Cardiovascular disease is a leading cause of death and disability. One cardiovascular disease that affects a large number of people is hypertension, which is defined as abnormally elevated blood pressure. Hypertension is quite common. It is estimated that over 60,000,000 Americans suffer from hypertension.
To prevent cardiac disorders from causing death, serious illness and disability, it is important to monitor the condition of a person's cardiovascular system, and to analyze the data from the monitoring so performed to determine whether any pathologies exist in the person's cardiovascular system that should be treated to prevent further degradation of the patient's cardiovascular system.
The method used most often to monitor a cardiovascular condition is the determination of the blood pressure of the patient. Human blood pressure is normally described by systolic and diastolic pressure readings, which are usually given in millimeters of mercury (mmHg). The systolic pressure is the higher of the two values given, and the diastolic pressure is the lower of the two values given. From a physiologic standpoint, the systolic pressure usually represents that pressure at which blood begins flowing through an artery that is compressed by a blood pressure cuff during a blood pressure measurement. At pressures above the systolic pressure (supra-systolic pressures) the flow of blood through the artery is blocked by the blood pressure cuff used to take the blood pressure reading. The diastolic pressure is that pressure below which the blood flow through the artery is unimpeded by the blood pressure cuff. A further explanation of the physiologic basis of the systolic and diastolic blood pressure readings can be found in Chio, U.S. Pat. No. 4,880,013, that issued on Nov. 14, 1989, and Chio U.S. Pat. No. 5,162,991, that issued on Nov. 10, 1992. The Chio '013 and '991 patents were invented by the Applicant, and are assigned to the assignee of this application.
It is generally accepted that a systolic blood pressure reading of greater than 140 mmHg, and/or a diastolic blood pressure reading of greater than 90 mmHg is indicative of a hypertensive condition. These pressure readings are generally considered to be indicative of hypertension, regardless of whether these blood pressure readings are made by non-invasive or invasive blood pressure determination methods.
Although systolic blood pressure and diastolic blood pressure readings are useful for determining whether hypertension exists, they are not completely reliable. The systolic/diastolic hypertension threshold (140 mmHg/90 mmHg) line of demarcation does not always provide a completely accurate guide for determining either which patients are hypertensive, or what factors caused the hypertension. In this regard, it is believed that approximately 80% of hypertension cases are categorized as "essential hypertension." A diagnosis of "essential hypertension" usually means that the causes of the hypertension are unknown. As such, these persons having "essential hypertension" may not be diagnosed accurately and reliably by only measuring the patient's systolic and diastolic pressures. For example, a patient may have a measured systolic and diastolic pressure of less than 140 (systolic)/90 (diastolic), but still may be genetically hypertensive. Conversely, a person may have a measured systolic/diastolic blood pressure of greater than 140/90, but may be not hypertensive either through environment, or genetic causes. Most importantly, it is difficult, if not impossible for a physician to treat a patient's hypertension properly if the physician does not know the cause of the hypertension.
For more than twenty years, studies have been conducted to find other physiological hemodynamic parameters in addition to systolic and diastolic blood pressure readings. For example, in the mid-1970's, Watt performed studies that tried to evaluate the "compliance" or "elasticity" of an artery. Watt, T. B. at et al., Arterial Pressure Contour Analysis for Estimating Human Vascular Properties, J. Applied Physics, (1976); at pages 171-176. In Watt's study, he used an electrical circuitry model, and a Windkessel model that were modified for a human arterial system to make his model for determining physiological and hemodynamic parameters. Watt's model defined two compliance components, C.sub.1 and C.sub.2, a Resistance, R and an Inductance, L. By using equations that had their genesis in the electrical circuitry art area, Watt further defined that C.sub.1 was the elastic compliance of major or large arteries. This factor (C.sub.1) was also called "proximal compliance." Watt found that C.sub.2 is the compliance of the smaller peripheral arteries, which is also referred to as "distal compliance."
Watt reported that correlations existed between the value of the proximal compliance (C.sub.1) and the distal compliance (C.sub.2) and the existence of hypertension. Primarily, Watt found that hypertensive patients tended to have smaller compliance values (C.sub.1 and C.sub.2). Since Watt's study, many other studies have been conducted that were focused on the arterial compliances and their relations to various causes of hypertension. Many groups have reported the relationship between proximal compliance (C.sub.1) and hypertension. In U.S. Pat. No. 5,054,493, which issued Oct. 8, 1991, J. N. Cohn, et al. reported his findings that distal compliance (C.sub.2) is more sensitive than proximal compliance (C.sub.1) for determining hypertension. Cohn therefore suggested that distal compliance (C.sub.2) was a better parameter for diagnosing hypertension than proximal compliance (C.sub.1). Cohn is also worth reviewing for its discussion of the Windkessel model, and its citation of a large number of references dealing with studies relating to compliance. At column 3, Cohn cites a larger number of studies conducted on the properties of the large proximal arteries, and the relationship of the properties of these arteries (in particular their compliance (C.sub.1)) to hypertension.
Since C.sub.2 is the distal compliance, and since distal compliance is strongly influenced by the reflection wave from the peripheral arteries in the arterial system, its measurement may need to be performed either by an invasive method, or alternately by a very sensitive non-invasive sensing device. An extremely sensitive non-invasive sensing device is probably necessary in order to obtain a near-perfect wave of the type that is typically found when using invasive techniques. This reflection phenomenon and its impact on its measurement was reported by Schwid, in Schwid, H. A., et al., Computer Model Analysis of Radial Artery Pressure Waveforms, J. Clinical Monitoring (1987), Vol. 3, No. 4, at pages 220-228. Additionally, the measurement of distal compliance (C.sub.2) may also be affected by the reflection wave. Further, the measurement of distal compliance may have fluctuations caused by other human factors, such as fluctuations in the arterial cross-section area and arterial blockage at the measured limb. As such, distal compliance C.sub.2 is still not a very reliable parameter for determining the physical conditions of a human cardiovascular system and other hemodynamic parameters. A recent study by Hayoz suggests that compliance may not be a valid indicia of hypertension, as Hayoz's study found that the elastic behavior (compliance) was not necessarily altered by an increase in blood pressure. See, Hayoz, D. et al., Conduit Artery Compliance and Distensibility are Not Necessarily Reduced in Hypertension, Hypertension 1992, Vol. 20, at pages 1-6.
Although the references cited above all relate to methods for determining cardiac and cardiovascular condition, and some of the methods discussed above relate to hemodynamic parameters other than the determination of systolic and diastolic pressure, room for improvement exists.
It is therefore one object of the present invention to provide an improved method for determining hemodynamic parameters in a human cardiovascular system.